Generally, pain is experienced when bodily tissues are subjected to mechanical, thermal or chemical stimuli of sufficient intensity to be capable of producing tissue damage. Pain resolves when the stimulus is removed or the injured tissue heals. However, under conditions of inflammatory sensitization or damage to actual nerve tissue, spontaneous pain may become chronic or permanent despite apparent tissue healing. Pain may be felt in the absence of an external stimulus and the pain experienced due to stimuli may become disproportionately intense and persistent.
Pain can take a variety of forms depending on its origin. Pain may be described as being peripheral neuropathic if the initiating injury occurs as a result of a complete or partial transection of a nerve or trauma to a nerve plexus. Alternatively, pain may be described as being central neuropathic following a lesion to the central nervous system, such as a spinal cord injury or a cerebrovascular accident. Inflammatory pain is a form of pain that is caused by tissue injury or inflammation. Following a peripheral nerve injury, symptoms are typically experienced in a chronic fashion, distal to the site of injury and are characterized by hyperesthesia (enhanced sensitivity to a natural stimulus), hyperalgesia (abnormal sensitivity to a noxious stimulus), allodynia (widespread tenderness, associated with hypersensitivity to normally innocuous tactile stimuli), and/or spontaneous burning or shooting lancinating pain.
Inflammatory pain has a distinct etiology, as compared to other forms of pain. After initiation of inflammation in peripheral tissues, functionally specialized primary afferent nerve fiber endings called nociceptors become sensitized resulting in the development of inflammatory pain. Examples include the pain that develops in association with inflammatory conditions such as arthritis, tendonitis and bursitis. Inflammatory pain may also arise from the viscera and an example is inflammatory bowel disease. Inflammatory pain is also often a component of cancer pain, post-operative pain, trauma pain and burns pain.
Inflammation-induced nociceptor sensitization leads to increased sensitivity and amplified responses so that pain may be produced by low-intensity or normally innocuous stimuli. Further, inflammatory pain involves neuroplastic changes at multiple levels of the nervous system including the nociceptors themselves, the dorsal root ganglia (DRGs), the dorsal horn of the spinal cord and the brain (Woolf and Costigan, 1999, Proc Natl Acad Sci USA 96: 7723-7730).
After initiation of inflammation, intracellular contents leak into the extracellular fluid, inflammatory cells are recruited and there is increased production and release of a broad range of pro-nociceptive (i.e. pro-pain) molecules e.g. protons, serotonin (5HT), histamine, adenosine, adenosine triphosphate (ATP), bradykinin, prostaglandinE2 (PGE2), nitric oxide (NO), interleukin-1 (IL-1), tumor necrosis factor alpha (TNFα), interleukin-6 (IL-6), leukemia inhibitory factor (LIF), nerve growth factor (NGF), by inflammatory and other cells. Exposure of nociceptors to this pro-inflammatory “soup” has the potential to cause sensitization so that innocuous stimuli are detected as painful (allodynia) or there is a heightened response to noxious stimuli (hyperalgesia) (Millan M J, 1999, Prog in Neurobiol 57: 1-164). This in turn initiates early post-translational changes in the nociceptors thereby altering transduction sensitivity (peripheral sensitization) which may in turn increase C-fiber activity producing subsequent sensitization of dorsal horn neurons (central sensitization). Both peripheral and central sensitization alter basal sensitivity to noxious and non-noxious stimuli (Woolf and Costigan, 1999, supra; Porreca et al., 1999, Proc Natl Acad Sci USA 96: 7640-7644). Additionally, there are other longer-lasting transcription-dependent changes in both the DRGs and the dorsal horn of the spinal cord involving the retrograde transport of specific signaling molecules e.g. nerve growth factor (NGF), which is produced as a result of inflammation. The net result is that inflammation results in a potentiated nociceptive signaling system as well as a system whereby phenotypic changes in low-threshold Aβ-fiber inputs have the potential to contribute to the development of stimulus-evoked rather than basal hypersensitivity (Woolf and Costigan, 1999, supra; Neumann et al., 1996, Nature (London) 384: 360-364).
Although nociceptors are defined by their normally high threshold for activation, lower intensity stimuli will activate sensitized nociceptors. Peripheral sensitization, which can be detected within a very short period, is thought to involve changes either in the transducing molecules/receptors themselves or in the Na+ channels in the nerve terminals (Woolf and Costigan, 1999, supra). A change in the transducer is best exemplified by the TRPV1 receptor, where repeated heat stimuli or exposure to protons progressively augments the inward current through the TRPV1 receptor ion channel (Caterina et al., 1997, Nature (London) 389: 816-24; Tominaga et al., 1998, J Neurosci 18: 10345-55). Additionally, phosphorylation of membrane-bound receptor/ion channels may occur as many inflammatory mediators activate protein kinases thereby increasing receptor phosphorylation. Phosphorylation of the peripherally located tetrodotoxin1-resistant (TTXr) sodium channels, results in a greater sodium current in the terminal (Gold et al., 1998, J Neurosci 18: 10345-10355; England et al., 1996, J Physiol (London) 495: 429-40; Gold et al., 1996, Proc Natl Acad Sci USA 93: 1108-12). These sensitizing changes occur locally in the peripheral nerve terminal, independent of any transcriptional changes that may occur in the neuronal cell bodies located in the DRGs. Tetrodotoxin is a neurotoxin from the puffer fish
Inflammation increases peripheral levels of NGF (Woolf et al., 1994, Neuroscience 62: 327-31), a neurotrophin thought to play a key role in inducing transcriptional changes such as upregulation of TRPV1-receptors and sensory-neuron-specific Na+-channels (Tate et al., 1998, Nat Neurosci 1: 653-55; Okuse et al., 1997, Mol Cell Biol 10: 196-207) in inflammatory states. Although peripheral sensitization does not itself require transcription, upregulated synthesis of components of the pain signaling system has the potential to amplify peripheral sensitization. After initiation of inflammation, there is a delay of many hours for up-regulated expression and transport of proteins to occur (Woolf and Costigan, 1999, supra).
Central sensitization of the spinal cord results in an NMDA receptor-sensitive increase in responsiveness to low- and high-intensity stimuli, both when applied to the site of inflammation (1° hyperalgesia) and in the contiguous non-inflamed area (2° hyperalgesia). Tactile allodynia and pin prick hyperalgesia in the zone of 2° hyperalgesia (Koltzenburg et al., 1992, Pain 51: 207-20) are characteristic NMDA receptor-mediated features of central sensitization (Stubhaug et al., 1997, Acta Anaesthesiol Scand 41: 1124-32). A consequence of inflammation-induced transcriptional changes in DRG neurons is that some low-threshold Aβ fibers may acquire the neurochemical phenotype typical of C-fibers such as synthesis and storage of substance P (Neumann et al., 1996, Nature 384: 360-364). This change in neurochemical expression together with the inflammation-induced increase in neurokinin-1 (NK-1) receptors in the dorsal horn of the spinal cord (Krause et al., 1995, Can J Physiol Pharmacol 73: 854-859) produce not only a potentiated system but one in which the specific type of stimulus that can evoke central sensitization has changed. Stimulus-induced hypersensitivity can thus be mediated by low-intensity Aβ inputs as well as high-intensity C-fiber inputs which manifests as progressive tactile allodynia where light touch produces a progressive increase in excitability of spinal cord neurons, something that would not happen in the non-inflamed state (Neumann et al., 1996, supra; Ma and Woolf, 1996, Pain 67: 97-106; Ma and Woolf, 1997, NeuroReport 8, 807-810; Ma and Woolf, 1997, Eur J Pharmacol 322: 165-171; Ma et al., 1998, Pain 77: 49-57).
Current methods for treating inflammatory pain have many drawbacks and deficiencies. For example, corticosteroids, which are commonly used to suppress destructive autoimmune processes, can result in undesirable side effects including, but not limited to, vulnerability to infection, weakening of tissues and loss of bone density leading to fractures, and ocular cataract formation. Non-steroidal anti-inflammatory drugs may cause gastrointestinal disturbances including ulceration and gastrointestinal bleeding, skin rashes and urticaria and interstitial nephritis. More recently, the cardiovascular safety of the selective cyclooxygenase-2 (COX-2) inhibitors has been raised as a potentially serious concern when these agents are administered chronically for periods longer than one year.
Thus, better therapeutic strategies are required for the treatment and management of inflammatory pain.